The European XFEL is a facility designed to generate ultra-short X-ray flashes at 27,000 times per second and with a high brilliance and beam energy, which is initially 12 keV (monoenergetic) but may be increased to higher energies. The facility will deliver very short pulses of spatially coherent laser light and the experimental end-stations will each contain additional light sources with different characteristics that combine to probe scientific samples. High speed detection of X-rays with customised instrumentation and a high dynamic range is highly desirable therefore presents a novel challenge.
A number of technologies exist in the area of high speed X-ray detection. For example, U.S. Pat. No. 7,868,665 describes a known hybrid sensor design, in which a CdZnTe detector is bonded to a readout Application Specific Integrated Circuit (ASIC) by Indium bump-bonding.
“Medipix3: A 64 k pixel detector redial chip working in single photon counting mode with improved spectra metric performance”, Ballabriga et al, Nuclear Instruments and Methods in Physics Research A 633 (2011) S15-S18, describes a hybrid pixel detector that uses a charge summing architecture, such that the charge in every cluster of four pixels is added and assigned to the pixel with the largest charge deposition asynchronously on an event-by-event basis. Data acquisition and readout can be performed either sequentially or continuously.
There are separate development programmes for customised systems specifically tailored for XFEL that are currently in development.
“Development of the LPD, a high dynamic range pixel detector for the European XFEL”, Hart et al, 2012 IEEE Nuclear Science Symposium and Medical Imaging Conference Record, P534-537 describes a sensor capable of a 4.5 MHz rate and high dynamic range. It is formed from 4096 pixel detector tiles with 500 μm pixels. The charge is read out by a custom ASIC. Eight ASICs are bonded to each sensor with 512 pixelated readout channels on each ASIC. A large and complex structure is needed to meet the demanding requirements.
“Electronics for the European XFEL: AGIPD a high frame rate camera”, P. Gottlicher, Topical Workshop on Electronics for Particle Physics 2010, IOP Publishing describes a detector array with electronics that automatically adds feedbacks capacitors to the input stage of the amplifier to avoid charge losses. Two capacitive pipelines store the analogue value and the used gain during the bunch data processing. The stored analogue value and gain for 1024 pixels are multiplexed into a single output line. Again, a large and complex design is necessitated.
Whilst these developments are on-going, in the separate area of UV and visible light imaging, new techniques have been developed for achieving higher frame rates. For example, our co-pending patent application GB1300190.4 (incorporated herein by reference in its entirety) and “Kirana: A Solid State Megapixel uCMOS image sensor for ultra-high speed imaging” Crooks et al, Proc. of SPIE Vol 8659, 865903 describe an active pixel sensor (APS) array of 30 μm pixels each containing a pinned photodiode, a set of 180 low-leakage storage cells, a floating-diffusion and a source follower output structure. In a fast mode, the storage cells are operated as a circular buffer, where 180 consecutive frames are stored until receipt of a trigger. In a ‘slow’ mode, the storage cell acts a pipeline and a sensor can be read out like a conventional sensor at a continuous frame rate of 1,180 fps. The sensor ASIC can be manufactured using a 180 nm CMOS technology. However, the Kirana sensor was not designed for X-ray applications.